The present invention relates generally to a method and apparatus for treating a subsurface earth formation penetrated by a well bore and, specifically, to a method and apparatus for pneumatically adding additional proppant to a pressurized fluid being injected into a well bore to stimulate a well.
Various methods are known for stimulating the production of crude oil and natural gas from wells drilled in reservoirs of low permeability. Certain prior art techniques have involved the hydraulic fracturing of such formations with various liquids such as crude oil, with or without proppants such as sand, glass beads, or the like. Hydraulic pressure was applied to the permeable formation to fracture the rock surrounding the well bore. The initially formed fractures were then extended by the injection of fluids containing a proppant to be deposited in the fractures. The hydraulic pressure was then released and the proppant which was deposited in the fractures served to hold the fractures open so that channels were created for flow of reservoir fluids to the well annulus. It has been recognized for some time that the concentration of proppant in the stimulation fluid is significant since it determines the final thickness of the fractures.
Another prior art technique for stimulating reservoirs of low permeability is the use of hydraulic fracturing with foam. Typical foam fracturing operations involve making a foam by blending sand with a jelled water solution and treating the solution thus formed with a surfactant. The fluid pressure is increased with a conventional pump after which a gas, such as nitrogen, is injected into the fluid to create a high pressure foam. The foam containing the sand proppant is then injected into the well. Foam fracturing has several advantages over fracturing techniques using conventional liquids. Foam has a low fluid loss and has the ability to create larger area fractures with equivalent volumes of treatment fluid. Since fluid loss to the formation is minimized, the chances of damaging sensitive formations is lessened. The foam is also thought to have a higher sand carrying and sand suspending capability to suspend a greater amount of sand in the foam until the fracture starts to heal. Since the foam has a high effective viscosity, sand does not settle out of the carrier fluid as quickly as it would settle from a traditional fluid such as crude oil. The foam creates wider vertical fractures as well as horizontal fractures of greater area.
In spite of the many advantages of using foam, some of which have been described, one disadvantage of such prior art techniques is that the maximum concentration of proppant obtainable is quite low. Conventional hydraulic fluids can achieve sand concentrations of 6 to 8 lbs. per gallon of carrying fluid. However, with foamed fluids, the gas expands the liquid to about four times the original volume of the gelled liquid. The net result is that the sand foam concentration is reduced to about 11/2 to 2 lbs. per gallon of carrying fluid.
In order to provide a foam fracturing fluid that was a high concentration of sand or other proppant, various schemes have been suggested which involve introducing the pressurized foamable fluid and sand slurry into a centrifugal separator or concentrator for separating some of the carrier fluid to concentrate the amount of proppant per volume of carrier fluid. The equipment necessary to effect such a separation is expensive and the separated fluid is usually wasted. Such schemes have generally been effective only to increase the proppant concentration from about 6 to 8 lbs. per gallon of carrying fluid to about 12 to 15 lbs. per gallon of carrying fluid. This results in a sand concentration of about 3 to 4 lbs. of proppant per gallon of carrying foam.
There exists a need, therefore, for a method and apparatus for treating a subsurface earth formation with pressurized foam which allows an increased proppant concentration to provide a greater fracture thickness.
There exists a need for such a method and apparatus which is simple in design and operation and which does not add greatly in the overall cost of the fracturing job.
There exists a need for such a method and apparatus which provides a proppant concentration in the foam carrier in excess of 4 lbs. per gallon of proppant carrier foam.
There exists a need for such a method and apparatus which provides a proppant concentration in a fluid in excess of 6 to 8 lbs. per gallon of carrying fluid without the wasted fluid and expense of separators or concentrators.
There exists a need for such a method and apparatus which reduces or eliminates the need for abrasive proppant passing thru and wearing very expensive conventional sand/fluid blending and high pressure pump equipment, and which reduces or eliminates the use of conventional sand/fluid blending and high pressure pump equipment.